1. Field of the Invention
This invention relates to improved polymers useful in molding containers such as bottles. More specifically, this invention relates to polyethylene terephthalate which is useful in making bottles which are employed in the carbonated beverage industry.
2. Summary of the Prior Art
The hazards of using glass containers, particularly glass bottles, for beer or carbonated beverages are well known. Breakage of such bottles often takes place due to the internal pressure exerted by the pressurized gas in the carbonated beverage or beer as well as by dropping the bottles and other impacts caused by external forces which occur not only in the course of production and distribution of the bottled product, but also as a result of handling of the bottled product by consumers. Such breakage may result in injuries to the human body.
Recently, the carbonated beverage industry has begun to use plastic, rather than glass, bottles for their carbonated beverages. Besides avoiding the hazards of using glass containers, the use of plastic, rather than glass, is advantageous in that plastic bottles are much lighter than glass bottles. Furthermore, less energy is required to make and transport plastic bottles.
Polyethylene terephthalate (hereinafter "PET") is a polymer which is well suited for such applications. PET may be prepared, as is well known, by the esterification of ethylene glycol and terephthalic acid or by the ester interchange of dimethyl terephthalate with ethylene glycol, followed by polycondensation in the presence of a catalyst such as antimony trioxide, at a temperature of 285.degree. C. and at a pressure of 1 millimeter of mercury. The PET product may then be extruded and pelletized. Unfortunately, these PET pellets cannot be used in the preparation of carbonated beverage bottles, because excessively large amounts of acetaldehyde are produced in the pellets under the conditions by which they are normally made. Furthermore, even if all of this residual acetaldehyde were removed from the PET pellets, it has been found that additional acetaldehyde is generated when a preform is molded from the PET, the molding occurring at temperatures generally above 240.degree. C. The presence of acetaldehyde usually affects the taste of any carbonated beverage, but particularly a cola flavored beverage which might be placed in such a container.
Accordingly, a commercially acceptable PET bottle must be prepared from PET which has insignificant amounts of acetaldehyde present and which does not generate significant additional amounts of acetaldehyde when heated for molding into a container or bottle.
It is also advantageous that the PET which is used in making the containers or bottle have a desirably high intrinsic viscosity, i.e., above about 0.60 deciliters per gram as calculated from measurements made on an 8% solution in o-chlorophenol at 25.degree. C. It is known that the intrinsic viscosity of PET may be increased by solid state polymerization in the presence of an inert gas such as nitrogen. In this connection, see, e.g., U.S. Pat. No. 4,064,112. The use of such inert gases in solid state polymerization processes is undesirable, however, because of economic considerations.
The search has continued for improved processes for reducing the acetaldehyde level and the generation rate of acetaldehyde in PET as well as for improved solid state polymerization processes wherein the intrinsic viscosity of PET is increased. This invention was made as a result of that search.